Preamble detector for a CDMA receiver

ABSTRACT

A method is provided of detecting one of a set of preamble sequences in a spread signal. The method includes: (a) correlating the received spread signal with sequences of a first orthogonal Gold code (OGC) set in accordance with a first fast transform to provide a preamble signal; (b) correlating the preamble signal with the set of preamble sequences in accordance with a second fast transform to generate a set of index values; (c) forming a decision statistic based on the set of index values: and (d) selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic. Step (c) comprises the steps of: 1) forming an initial decision statistic based on the relative maximum index of the set of index values; 2) selecting the signal generated by the preamble sequence combined with the preamble signal corresponding to the initial decision statistic; 3) adjusting, in one or more of amplitude and phase, the signal selected in step 2); and 4) forming the decision statistic based on the adjusted signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of European Patent Application No.99310117.9, which was filed on Dec. 15, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to signal demodulation in a code-division,multiple-access (CDMA) telecommunication system, and, more particularly,to despreading and preamble detection of code sequences in a CDMAreceiver.

2. Description of the Related Art

In wireless, cellular, or similar code-division, multiple-access (CDMA)telecommunication systems, multiple channels are defined for up-link(user to base station) and down-link (base station to user)communications. Generally, each channel includes a spread signalsynchronized to a common timing reference. A pilot signal is anunmodulated code sequence that is a transmitted throughout the systemfor synchronization of the various code sequences (in time and in codephase), and the pilot signal is used by the spreading (modulation) andde-spreading (demodulation) operations. Channel signals may be spread,in frequency, using pseudo-random (PN) and/or orthogonal code sequences.

Signals transmitted through a channel are binary data represented by asequence of values, each value termed a symbol. The spreading PN codesequence is also binary data represented by a sequence of values, eachvalue called a chip. However, the width (length in time) of a chip ismuch less than the width of a symbol. For some systems operating inaccordance with the IS-95 or wideband CDMA (W-CDMA) standards, eachsymbol may contain up to 256 chips. The spread signal may then becombined with an orthogonal code, such as a Walsh code (also known as aWalsh-Hadamaard sequence), to maintain relatively low to zerocross-correlation between multiple spread signals transmitted through atransmission medium. Each orthogonal code is also generated as asequence of binary values, the width of each value equivalent to thesymbol width.

Channels of these CDMA telecommunication systems may either be userchannels or system channels. Voice, voiceband data, or data signals aregenerally transmitted over user channels, while system channels are usedby a base station and users to broadcast information or are randomlyaccessed by users or the base station to establish a connection over achannel. The channels are usually spread using the same PN codesequences, and then each is spread with uniquely assigned orthogonalcode signal. A receiver may then differentiate the channels using theunique, orthogonal code signal that is assigned to each channel. Sincethe same PN code sequence is used for all channels, and the PN codesequence phase is readily determined with reference to the pilot signal,detection of the orthogonal code signal occurs only at the beginning ofthe user's session. The sessions of user channels are relatively long,so the detection process for each user occurs relatively infrequently.In addition, the base station and user have the following a prioriinformation: 1) the connection information when initialized, 2) theorthogonal code assigned to the user channel, and 3) coarsesynchronization information for the initial detection process.

Commonly used channels, broadcast channels, or randomly-accessedchannels, on the other hand, have bursty sessions that are relativelyshort, since only a small amount of information is exchanged between theuser and base station, for example, to initialize a connection. Inaddition, the receiver has no a priori information about the codesequence used, and the potential code sequence changes relatively often.Consequently, the orthogonal code detection process for these channelsmay occur frequently, and systems desirably minimize the time requiredto perform detection. For these channels characterized by burstysessions, a transmitter may use a preamble characterized by a signaturesequence to enable rapid detection and demodulation of spread signals.

For example, a burst of the Random Access Channel (RACH) in accordancewith W-CDMA standards is transmitted by the mobile user to initialize adedicated set up procedure or for transporting data across the airinterface (connectionless data transfer). A typical RACH burst 100 isshown in FIG. 1A and comprises preamble 102 and message 104. An idletime period of length 0.25 ms occurs between the preamble 102 and themessage 104 that allows for detection of the preamble information(preamble signature sequence) and subsequent on-line processing of themessage. The transmission time of each RACH burst 100 is randomlyselected by the mobile according to 8 specified offsets, as shown inFIG. 1B, to increase throughput of the RACH data.

A preamble detector may employ the following a priori information todetect a given RACH preamble spreading (code number) and/or signaturesequence. First, a user randomly selects one sequence out of a code setallocated to the specific sector. Second, the sequences are known butthe number or subset available for use may vary depending on the trafficload of the sector. Third, the receiver (including the preambledetector) acknowledges multiple requests from RACH bursts occurringduring the same frame offset.

A CDMA receiver may employ non-coherent demodulation of the signal of auser channel. For systems conforming to the IS-95 standard, for example,the receiver may use 64-ary, non-coherent demodulation (despreading) ofchannels. For this case, a demodulator generates all of the possible(i.e., 64) orthogonal code sequences and combines each with the receivedsignal. A CDMA receiver may also employ coherent demodulation of thesignal of a channel. In coherent demodulation, an initial estimate ofthe assigned code signal for the channel is determined from the received(actual code) signal. Using the initial estimate, a signal representingthe error between the actual coded signal and the transmit code signalis employed to correct amplitude and phase deviations of the actual codesignal.

Detection of the RACH burst preamble spans three axes. The first axis isthe time axis that evaluates the energy content of hundreds of Tc/2delay cells (Tc is the chip duration). The exact length of theuncertainty area depends on the size of the cell. The second axis is thecode axis that evaluates these delay cells for all permissible spreadingcodes. The third axis is the frequency/phase uncertainty axis, whichaxis is related to the detection statistic (e.g., a non-coherent,quasi-coherent, or coherent decision statistic) used to determine therelative likelihood that the correct code with correct frequency andphase is detected.

SUMMARY OF THE INVENTION

The present invention relates to detection of one of a set of preamblesequences in a spread signal where either the preamble sequence is anorthogonal code sequence, the spreading sequence of the spread signal isan orthogonal code sequence, or both the preamble and spreadingsequences are orthogonal code sequences. The orthogonal code sequencesmay be selected from either a set of Walsh-Hadamaard sequences or a setof orthogonal Gold code (OGC) sequences. Where the orthogonal codesequences are selected from the set of Walsh-Hadamaard sequences, adetector and/or demodulator may employ a fast Hadamaard transform (FHT)method for signal correlation. Where the orthogonal code sequences areselected from the set of OGC sequences, a detector and/or demodulatormay employ a fast transform method based on the method used to generatethe OGC sequences, the fast transform method including the FHTtransform.

In accordance with an embodiment of the present invention, a preambledetector correlates the received spread signal with sequences of a firstorthogonal Gold code (OGC) set in accordance with a first fast transformmethod to provide a preamble signal, wherein the spread signal is thepreamble sequence combined with one sequence of the first OGC set. Thepreamble signal is then correlated with the set of preamble sequences inaccordance with a second fast transform method to generate a set ofindex values. A decision statistic is formed based on the set of indexvalues, and the preamble sequence corresponding to the decisionstatistic is selected as the detected one of the set of preamblesequences.

For another embodiment, the received spread signal is correlated with aset of orthogonal sequences to provide a preamble signal, wherein thespread signal is the preamble sequence combined with one sequence of theset of orthogonal sequences. The preamble signal is correlated with oneor more preamble sequences of an orthogonal Gold code (OGC) set inaccordance with a fast transform method to generate a set of indexvalues. A decision statistic is formed based on the set of index values;and the preamble sequence corresponding to the decision statistic isselected as the detected one of the set of preamble sequences.

For some exemplary embodiments, either the spreading sequence orpreamble sequence is selected from the OGC set formed from first andsecond sequence vectors, wherein the OGC set is generated from the firstsequence vector and a cyclic shift matrix of a second sequence vector.The fast transform method is a fast orthogonal Gold code transform(FOGT) that comprises the following steps. First, the preamble signal ismultiplied with a first sequence vector and a forward permutation vectorto generate a permuted preamble signal, the forward permutation vectormapping between i) the cyclic shift matrix and ii) a matrix ofWalsh-Hadamaard sequences. Second, the fast Hadamaard transform isapplied to the permuted preamble signal to generate the set of indexvalues.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claims, and the accompanying drawings in which:

FIG. 1A shows a random access channel (RACH) burst of the prior artcomprising a preamble and a message;

FIG. 1B shows a transmission timeline for offsets of RACH bursts asshown in FIG. 1A;

FIG. 2 shows a signal flow block diagram of a RACH burst signalincluding a preamble detector operating in accordance with an exemplaryembodiment of the present invention;

FIG. 3 is an exemplary flow chart for preamble correlation in accordancewith the present invention;

FIG. 4 shows a preamble detector operating in accordance with anexemplary embodiment of the present invention generating a coherentdetection statistic;

FIG. 5 shows an alternative embodiment of the preamble detector shown inFIG. 4;

FIG. 6 is a graph of the probability that the incorrect decoded signalof the preamble detector of FIGS. 4 and 5 will be selected in a singlepath Rayleigh fading channel; and

FIG. 7 is a graph of the envelope of the decoded signal of the preambledetector of FIGS. 4 and 5 that fluctuates according to the channelvariations.

DETAILED DESCRIPTION

In accordance with the present invention, a code-division,multiple-access (CDMA) preamble detector applies either a Fast HadamaardTransform (FHT) or a Fast Orthogonal Gold-code Transform (FOGT) to thereceived spread signal for either of the despreading and preamblesignature sequence detection processes. Implementations operating inaccordance with the present invention may allow for relatively simpledetection circuits. Detection of preamble sequences may allow rapiddetection and demodulation of data signals in channels randomly accessedby multiple users in a CDMA telecommunication system, and such channelsmay be, for example, the random access channel (RACH) of systemsoperating in accordance with wideband-CDMA (W-CDMA) standards.

For the exemplary implementations of the present invention as describedbelow, a CDMA receiver detects the preamble part of a random accesschannel (RACH) burst, although the techniques as described herein may bereadily extended to other CDMA applications. The RACH burst may be asshown in FIGS. 1A and 1B. In accordance with the exemplary embodimentsdescribed herein, the preamble preferably comprises a signature sequenceof 16 complex symbols (±1±j) (i.e., signature sequence of length 16). Atotal of 16 different signature sequences may exist and all belong to anOrthogonal Gold Code (OGC) set of length 16. As would be apparent to oneskilled in the art however, other sequence lengths (other than length16) and other types of orthogonal code sequences may be employed, suchas Walsh code sequences.

Each symbol of the preamble signature sequence is spread with a 256-chipcode sequence from an OGC set of length 256. Consequently, 256 codessequences are available (code numbers C1, . . . ,C256) and are allocatedthrough the CDMA network for spreading of channels. The code numbersthat a user may select are transmitted in the sector-specific broadcastchannel (BCCH). The user randomly selects one of the permissible codesand one of the 16 preamble signatures, and uses the selected signaturesequence to create the preamble of a RACH burst.

Once the received signal is despread, preamble detection in accordancewith exemplary implementations of the present invention correlatessignature sequences against the despread input vector (i.e., signalsamples) using fast transform techniques, and then forms a decisionstatistic to identify the preamble signature sequence. If coherent orquasi-coherent detection is employed, preamble detection corrects forphase or amplitude variations (if coherent or quasi-coherent detectionis employed) to create the final decision statistic. Once the preambleis detected, demodulation and data detection of the message part of theRACH burst may be in accordance with techniques well known in the art.

FIG. 2 shows a block diagram representing signal flow for the encoding,transmission, and detection of a RACH burst signal. As shown in FIG. 2,OGC generator 201 provides an OGC sequence (spreading code) based on aninput code number. Mapping operator 202 converts the binary values ofthe OGC sequence from “1” or “0” to “−1” or “1”, respectively, and theOGC sequence is then combined with the preamble signature sequence incombiner 203. The signal from combiner 203 input to the modulator 204 istransmitted as a complex signal (in-phase (I) and quadrature (Q)components) through a transmission channel that is a fading channel withadditive white, Gaussian noise (AWGN). The transmission channel may berepresented as shown in FIG. 2 for convenience of the followingdescription as blocks 205 and 206. Demodulator 207 receives the signaland provides the received spread data signal (r_(I) and r_(Q)) to codematched filter and despreader (CMF/despreader) 208. CMF/despreader 208correlates the received spread data signal with members of the OGC setemploying the FOGT transform (described subsequently) to despread theRACH preamble. CMF/despreader 208 provides the despread data signal assymbols of the preamble signature sequence, which are then correlatedwith the set of signature sequences by preamble sequence detector 209 togenerate a decision statistic used to identify the signature sequence.Preamble sequence detector 209 may employ the FOGT for correlation ifmembers of an OGC set are used for signature sequences, or may employthe fast Hadamaard transform (FHT) that is well known in the art ifWalsh-Hadamaard sequences are used for signature sequences. In eithercase, the correlation provides a peak value as a decision statistic. Forthe following description, the real component operations are described,but one skilled in the art may extend the following to complex signals.

The FOGT transform is related to the FHT transform. The FOGT transformmay be extended to sequences that belong to any OGC set. Orthogonal Goldcode sequences may be generated as a base-2 Galois field (GF(2))addition of selected pairs of pseudo-random code sequences(m-sequences). The length, or period, of an m-sequence is defined as N,where N is 2^(L)−1 and L is the length of the shift register generatingthe m-sequence (N, L are positive integers). M is defined as the periodof each OGC sequence, and the OGC sequence length may be an even lengthto align chip code periods with data symbol periods. Since an m-sequencehas a length that is an odd number, each OGC sequence is generallyaugmented by one value (e.g., chip or symbol) to provide a sequencehaving an even length with M being N+1=2^(L). The OGC set {OGC (a,b)}that is produced by two m-sequences a and b is given in equation (1):OGC(a,b)={a, b, Da⊕b, D ² a⊕b, . . . , D ^(N-1) a⊕b}  (1)where the operator D^(x) denotes a cyclic shift of the m-sequence a by xpositions. A member of the OGC set is not a cyclic shift of anothermember in the OGC set. A delay of y chips in the OGC sequence may beproduced by computing the delays in the corresponding m-sequencegenerators and initializing the m-sequence generators accordingly.

The OGC set employed in accordance with the exemplary embodiment of thepresent invention for either spreading the preamble or the signaturesequence may be generated as follows. First, the matrix X is formed asshown in equation (2): $\begin{matrix}{X = {\begin{bmatrix}\underset{\_}{0} & \underset{\_}{0} \\\underset{\_}{0} & F\end{bmatrix} = \left\lbrack {\begin{matrix}0 & 0 \\0 & a_{1} \\\vdots & \vdots \\0 & a_{N}\end{matrix}\begin{matrix}0 & \ldots \\a_{2} & \ldots \\\vdots & \ldots \\a_{1} & \ldots\end{matrix}\begin{matrix}0 & 0 \\a_{N - 1} & a_{N} \\\vdots & \vdots \\a_{N - 2} & a_{N - 1}\end{matrix}} \right\rbrack}} & (2)\end{matrix}$where F is the matrix comprising all the cyclic shifts of the m-sequencea with period N. For an OGC set of length 256, for example, them-sequence a is generated by the polynomial given in equation (3):a(D)=1+D ² +D ³ +D ⁴ +D ⁸  (3)where D⁰ is 1, or by the recursiona_(i)=a_(i-8)+a_(i-6)+a_(i-5)+a_(i-4). The matrix S_(xa) is formed byadding the “0” row and column to the matrix F. Adding the “0” row andcolumn accounts for the augmentation of the sequence by one value toprovide a sequence of even length.

Second, the column vector b_(x) is formed as given in equation (4):b_(x)=[0 b]^(T)  (4)where the vector b is the m-sequence b. The m-sequence b for an OGC setof length 256 may be generated by the polynomial given in equation (5):b(D)=1+D ³ +D ⁵ +D ⁶ +D ⁸  (5)or by the recursion b_(i)==b_(i-8)+b_(i-5)+b_(i-3)+b_(i-2). The columnvector b_(x) is the vector b augmented by a chip value of “0” to providea sequence of even length.

Third, the matrix C is formed by addition over GF(2) each row of matrixS_(xa) and vector b_(x) as given by equation (6):

 C=S _(xa) ⊕b _(x)  (6)

For example, each spread RACH preamble symbol may be represented by oneof the rows of the 256×256 matrix C derived from equation (6) as givenin equation (7): $\begin{matrix}{C = \left\lbrack {\begin{matrix}c_{11} & c_{12} \\c_{21} & c_{22} \\\vdots & \vdots \\c_{M1} & c_{M2}\end{matrix}\begin{matrix}\ldots \\\ldots \\\vdots \\\ldots\end{matrix}\begin{matrix}c_{1M} \\c_{2M} \\\vdots \\c_{MM}\end{matrix}} \right\rbrack} & (7)\end{matrix}$

It is well known in the art of communication theory that the optimumdetector (or despreader) of a code sequence in AWGN may be realized as abank of Code Matched Filters (CMFs), where each CMF may be implementedwith an FIR filter matched to a corresponding cyclic shift of the code.For a random propagation delay of x chips where the propagation delaydoes not exceed the period of the code, the CMF matched to the sequenceD^(x)c_(k) indicates a peak level (i.e., a “hit”) once every codeperiod. A bank of CMFs may be employed, with each CMF of the bank beingused to detect a different corresponding code sequence (e.g., M-codesdetected with uncertainty of M-chips may require M² CMFs).

A transmitted signal with additive white Gaussian noise (AWGN) that isreceived at a transceiver (e.g., CMF/despreader 208) may be defined byequation (8):r=Cp _(s) +n  (8)where r is the received vector, C is the OGC matrix with “0”s mapped to“1”s and “1”s mapped to “−1”s, p_(s) is the symbol of the preamblesignature sequence, and n is the noise vector. For the followingdescription, the real component is described, although one skilled inthe art may extend the following to complex signal processing.

FIG. 3 is an exemplary flow chart for the method of block correlationfor preamble detection in accordance with embodiments of the presentinvention. The method of preamble detection comprises three steps: step301, step 302, and step 303. At step 301, given the relationship betweenthe matrix C and b_(x) of equations (6) and (7), the received inputvector r is correlated with (i.e., multiplied by) the vector b_(x)(augmented m-sequence b). The vector b_(x) is constant since the initialvalue of the feedback register is constant (e.g., always FF hex).

At step 302, the resulting sequence from the combination of r and b_(x)is correlated with the inverse of the circular matrix C using a forwardpermutation vector and a fast Hadamaard transform (FHT). The correlationoperation with the inverse of C may be implemented as follows. Thepermutation vector π_(f) is applied to the combination of r and b_(x) atstep 302 a. Arranging the sequence that results from the combination ofr and b_(x) with the forward permutation vector π_(f) generates apermuted sequence compatible with block correlation of the FHTtransform.

To calculate the permutation vector π_(f), the matrix F is augmented forthe additional chip value to form the matrix X as given in equation (9),$\begin{matrix}{X = \begin{bmatrix}\underset{\_}{0} & \underset{\_}{0} \\\underset{\_}{0} & F\end{bmatrix}} & (9)\end{matrix}$

The forward permutation vector π_(f) may be computed as follows given amatrix F having an augmented column and row (such as in equation (9)).For sequences of length M=2^(L) (M, L integers greater than 1), apermutation vector π_(f) of length L exists that rearranges the rows(columns) of the matrix X, such that the columns (rows) in the resultingmatrix W are Walsh-Hadamaard sequences. The forward permutation vectorπ_(f) may be defined as given in equation (10):π_(f)=[i_(l), . . . , i_(L)]  (10)For example, the entries of the forward permutation vector π_(f)identify the mapping of rows to the matrix W from a matrix X. Since theforward permutation vector π_(f) remains constant it may be computedonce through recursive search techniques and stored. Consequently,implementations that apply the permutation vector π_(f) may use alook-up table (LUT).

The method may be shown using sequences of length M=2³ (i.e., L=3) andby considering a polynomial with a smaller degree, e.g., f(x)=1+x+x³ forthe m-sequence. For an initial load of [1 1 1], the m-sequence generatorprovides sequence values in accordance with equation (11),x(n)=x(n−3)+x(n−2)  (11)to generate the sequence [1 1 1 0 0 1 0]. The F-matrix formed withcyclic shifts of the m-ary sequence a is then generated as given inequation (12): $\begin{matrix}{F = \begin{bmatrix}1 & 1 & 1 & 0 & 0 & 1 & 0 \\1 & 1 & 0 & 0 & 1 & 0 & 1 \\1 & 0 & 0 & 1 & 0 & 1 & 1 \\0 & 0 & 1 & 0 & 1 & 1 & 1 \\0 & 1 & 0 & 1 & 1 & 1 & 0 \\1 & 0 & 1 & 1 & 1 & 0 & 0 \\0 & 1 & 1 & 1 & 0 & 0 & 1\end{bmatrix}} & (12)\end{matrix}$and the matrix F is extended to be the matrix X (from equation (9)) asgiven in equation (13): $\begin{matrix}{X = \begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 1 & 1 & 1 & 0 & 0 & 1 & 0 \\0 & 1 & 1 & 0 & 0 & 1 & 0 & 1 \\0 & 1 & 0 & 0 & 1 & 0 & 1 & 1 \\0 & 0 & 0 & 1 & 0 & 1 & 1 & 1 \\0 & 0 & 1 & 0 & 1 & 1 & 1 & 0 \\0 & 1 & 0 & 1 & 1 & 1 & 0 & 0 \\0 & 0 & 1 & 1 & 1 & 0 & 0 & 1\end{bmatrix}} & (13)\end{matrix}$

The permutation vector is π_(f)=[1 7 4 3 6 2 8 5], where “1” is thefirst row of X mapped to the first row of W, “5” is the fifth row of Xmapped to the second row of W, and so on. The row permutations of thematrix X according to the forward permutation vector π_(f) are employedto create the matrix W given in equation (14): $\begin{matrix}{W = \begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 0 & 1 & 1 & 1 & 0 \\0 & 1 & 0 & 0 & 1 & 0 & 1 & 1 \\0 & 1 & 1 & 0 & 0 & 1 & 0 & 1 \\0 & 0 & 1 & 1 & 1 & 0 & 0 & 1 \\0 & 0 & 0 & 1 & 0 & 1 & 1 & 1 \\0 & 1 & 1 & 1 & 0 & 0 & 1 & 0 \\0 & 1 & 0 & 1 & 1 & 1 & 0 & 0\end{bmatrix}} & (14)\end{matrix}$

As would be apparent to one skilled in the art, the orthogonal matrix Wis a matrix with columns representing Walsh-Hadamaard sequences of the8×8 Walsh-Hadamaard matrix, and so the matrix W may be block correlatedusing the FHT transform. Consequently, in accordance with the exemplaryembodiment, the input sequence (the combination of r and b_(x)) may, forthe ideal channel, be a row of the matrix X, and applying the forwardpermutation vector π_(f) maps the input sequence into a correspondingcolumn of the matrix W.

Returning to FIG. 3, the resulting sequence of step 302 a is thenprocessed with the fast Hadamaard transform (FHT) at step 302 b. Forstep 302 b, the FHT (e.g., the 256-point FHT) is applied to the permutedsequence (i.e., combination of r and b_(x) arranged in accordance withthe permutation vector π_(f)) to generate the output vector z (for theexemplary implementation with OGC sequences of length 256, z=(z₁, z₂, .. . , z₂₅₆)). The output vector z corresponds to the block correlationof the signal sequence in W with the members of the OGC set.

At step 303, a reverse permutation vector π_(r) is applied to the outputvector z to generate the correlated output sequence. Given the outputvector z, the index of the vector z is mapped to the corresponding OGCnumber. The reverse permutation vector π_(r) for this mapping iscalculated based on the one-to-one relationship between the code orsignature number and the index of the correlation peak in the FHT outputvector z from the block correlator (e.g., CMF/despreader 208 in FIG. 2).

For an OGC set used for spreading (e.g., OGC set of length 256) and anOGC set used for signature sequences (e.g., OGC set of length 16), thereceived signal is despread using the sequence of the code number, thenthe despread signal is applied to the preamble sequence detector 209 inFIG. 2. The despread signal may again be processed in accordance withthe FOGT transform (i.e., have a forward permutation vector π_(r)applied, be FHT processed, and then have a reverse permutation vectorπ_(r) applied). However, for preamble detection, information of both thein-phase (I) and quadrature-phase (Q) channels may be employed.

The despread I and Q channel signals (r_(I) and r_(Q)) may be split intotwo branches for processing: a reference branch and a data branch. Thereference branch is employed for channel estimation and frequencyacquisition, if required. For example, the squared magnitude complex FHTvector signal of length 16 is calculated and the maximum index i, wherei, {1, . . . 16}, is selected for a non-coherent preliminary decisionstatistic. The index i may then be used by a Walsh generator toreproduce the bi-phase shift-keyed (BPSK) signature sequence, a selectorto select one of the 16 FHT outputs for coherent detection, or as thedecision statistic itself for incoherent detection. Exemplaryembodiments of a coherent RACH preamble detector are shown in FIGS. 4and 5.

FIG. 4 shows a preamble detector 400 operating in accordance with anexemplary embodiment of the present invention using coherent detection.The preamble detector 400 may be included in a demodulator for a randomaccess channel (RACH) of a code-division, multiple-access (CDMA)telecommunication receiver. The input signal is first applied to acode-matched filter (CMF) 401 that is matched to the spreading codesequence of the input signal. For example, in accordance with an IS-95standard, the code may be an OGC sequence at a spreading chip weight.The output of the CMF 401 is the despread preamble signal sampled at thesymbol rate, which is the rate of the Walsh-Hadamaard code sequenceemployed as the signature sequence for the RACH.

The output of the CMF 401 is correlated by an FHT or FOGT correlator 402to block correlate the despread preamble signal with the signaturesequence. The signal from FHT or FOGT correlator 402 is then provided totwo paths. For the exemplary embodiments described herein, the FHT orFOGT correlator 402 provides a group of either FHT or FOGT code-words oflength 16 matching the 16 Walsh code or OGC signature sequences,respectively, that may be transmitted by a user. Each of the decodedsignals from FHT or FOGT correlator 402 is provided in the second pathto a magnitude-squared operator 403 and the output provided to themaximum valued index (max (I)) operator 404. The max (I) operator 404determines which decoded signal of FHT or FOGT correlator 402 has thegreatest magnitude squared value corresponding to the index of thesignature sequence of the preamble.

Based upon the output of the max (I) operator 404, a selector (SEL) 405selects the decoded signal from FHT or FOGT correlator 402 having thecorresponding index value. SEL 405 provides the selected, decoded signalto the channel estimator (CHEST) 406. CHEST 406 provides an estimate ofthe amplitude and the phase signal for derotating the selected, decodedsignal and provides the estimate to combiner 407. Combiner 407 combinesthe estimate for the in-phase channel and the estimate for thequadrature-phase channel with the corresponding in-phase andquadrature-phase channels of the decoded signal in the first path fromFHT or FOGT correlator 402. The derotated signal provided by combiner407 is magnitude squared in operator 408 and the output provided as thefinal decision statistic L(Y). The final decision statistic L(Y) maythen be used by a demodulator, controller, or other processing circuitryto identify and synchronize demodulation of the RACH burst information.

FIG. 5 shows an alternative embodiment of the preamble detector 500operating in accordance with the present invention with Walsh codes usedfor signature sequences. The input signal is applied to code matchedfilter (CMF) 501 and the output signal is then provided to two paths. Inthe first path, the output of the CMF 501 is applied to combiner 506. Inthe second path, the output of the CMF 501 is applied to FHT correlator502. The FHT 502 provides, for example, 16 decoded (fast Hadamaardtransformed) signals generated from the output of the CMF 501. Each ofthe decoded signals of FHT correlator 502 is applied to the magnitudesquared operator 503 that provides magnitude value for eachcorresponding decoded signal. The max (I) operator 504 determines theindex of the Walsh code sequence corresponding to the decoded signalfrom FHT 502 having the greatest magnitude squared value. The indexprovided by max (I) operator 504 is then provided to a Walsh generator505 which then selects an appropriate Walsh code sequence and itsconjugate signal for combining with the output of CMF 501 in combiner506. An integrator 507 integrates the output signals for each of thein-phase and quadrature-phase channels provided from the combiner 506.The integrated signal is then provided to a magnitude-squared operator508 that provides a magnitude-squared value for the input signal. Thisoutput value from magnitude squared operator 508 is then used as thefinal decision statistic L(Y).

Simulation results may be employed to show expected performance ofimplementations of preamble detectors of FIGS. 4 and 5 operating inaccordance with the present invention. FIG. 6 is a graph showing theprobability that an incorrect decoded signal (i.e., the output of FHTcorrelator) will be selected in a single path Rayleigh fading channel(Jakes model, with v-120 km/hr). FIG. 7 is a graph showing the envelopeof the decoded signal (output of FHT correlator) that fluctuatesaccording to the channel variations. The I and Q components of thisdecoded signal may be used to estimate the channel characteristics with,for example, low pass filtering.

While the exemplary embodiments of the present invention have beendescribed with respect to processes of circuits, the present inventionis not so limited. As would be apparent to one skilled in the art,various functions of circuit elements may also be implemented in thedigital domain as processing steps in a software program. Such softwaremay be employed in, for example, a digital signal processor,micro-controller or general purpose computer.

The present invention can be embodied in the form of methods andapparatuses for practicing those methods. The present invention can alsobe embodied in the form of program code embodied in tangible media, suchas floppy diskettes, CD-ROMs, hard drives, or any other machine-readablestorage medium, wherein, when the program code is loaded into andexecuted by a machine, such as a computer, the machine becomes anapparatus for practicing the invention. The present invention can alsobe embodied in the form of program code, for example, whether stored ina storage medium, loaded into and/or executed by a machine, ortransmitted over some transmission medium, such as over electricalwiring or cabling, through fiber optics, or via electromagneticradiation, wherein, when the program code is loaded into and executed bya machine, such as a computer, the machine becomes an apparatus forpracticing the invention. When implemented on a general-purposeprocessor, the program code segments combine with the processor toprovide a unique device that operates analogously to specific logiccircuits.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the principle andscope of the invention as expressed in the following claims.

1. A method of detecting one of a set of preamble sequences in a spreadsignal comprising the steps of: (a) correlating a received spread signalwith sequences of a first orthogonal Gold code (OGC) set in accordancewith a first fast transform to provide a preamble signal; (b)correlating the preamble signal with the set of preamble sequences inaccordance with a second fast transform to generate a set of indexvalues, wherein each index value of the set of index values correspondsto a one preamble sequence of the set of preamble sequences; (c) forminga decision statistic based on the set of index values; and (d)selecting, as the detected one of the set of preamble sequences, apreamble sequence corresponding to the decision statistic; wherein step(c) comprises the steps of: 1) forming an initial decision statisticbased on the a relative maximum index of the set of index values; 2)selecting a signal generated by the preamble signal combined with thepreamble sequence corresponding to the relative maximum index of theinitial decision statistic; 3) adjusting, in one or more of amplitudeand phase, the selected signal in step 2); and 4) forming the decisionstatistic based on the adjusted selected signal.
 2. The invention asrecited in claim 1, wherein, for step (a), the first fast transformmethod is a fast orthogonal Gold code transform (FOGT) comprising thesteps of 1) multiplying the received spread signal with a first sequencevector and a forward permutation vector to generate a permuted sequencesignal, wherein: the first OGC set is generated from the first sequencevector and a cyclic shift matrix of a second sequence vector, and theforward permutation vector maps between i) the cyclic shift matrix ofthe second sequence vector and ii) a matrix of Walsh-Hadamaardsequences; and 2) applying a fast Hadamaard transform to the permutedsequence signal to generate a set of correlated signals, the preamblesignal selected as one of the set of correlated signals based on apredetermined decision criterion.
 3. The invention as recited in claim1, wherein: for step (b), the set of preamble sequences are selectedfrom a second OGC set formed from first and second sequence vectors, thesecond OGC set generated from the first sequence vector and a cyclicshift matrix of the second sequence vector; and wherein the second fasttransform is a fast orthogonal Gold code transform (FOGT) comprising thesteps of 1) multiplying the preamble signal with the first sequencevector and a forward permutation vector to generate a permuted preamblesignal, the forward permutation vector mapping between i) the cyclicshift matrix of the second sequence vector and ii) a matrix ofWalsh-Hadamaard sequences, and 2) applying a fast Hadamaard transform tothe permuted preamble signal to generate the set of index values.
 4. Theinvention as recited in claim 1, wherein, for step (b), the set ofpreamble sequences are selected from set of Walsh-Hadamaard sequences,and the second fast transform is a fast Hadamaard transform.
 5. Theinvention as recited in claim 1, wherein, for step (a), the receivedspread signal is a burst of a random-access channel in a code-division,multiple-access communication system.
 6. The invention as recited inclaim 1, wherein step (c3) adjusts the selected signal by estimating achannel response from the preamble signal, forming a de-rotation signalfrom the preamble signal, and combining the de-rotation signal with thepreamble signal for coherent sequence detection.
 7. The invention asrecited in claim 1, wherein step (c2) employs the initial decisionstatistic to locally generate a corresponding preamble sequence, thelocally generated preamble sequence being combined with the preamblesignal for coherent sequence detection.
 8. A method of detecting one ofa set of preamble sequences in a spread signal comprising the steps of:(a) correlating a received spread signal with a set of orthogonalsequences to provide a preamble signal; (b) correlating the preamblesignal with one or more preamble sequences of an orthogonal Gold code(OGC) set in accordance with a fast transform to generate a set of indexvalues, wherein each index value of the set of index values correspondsto a one preamble sequence of the set of preamble sequences; (c) forminga decision statistic based on the set of index values; and (d)selecting, as the detected one of the set of preamble sequences, apreamble sequence corresponding to the decision statistic; wherein step(c) comprises the steps of: 1) forming an initial decision statisticbased on a relative maximum index of the set of index values; 2)selecting a signal generated by the preamble signal combined with thepreamble sequence corresponding to the relative maximum index of theinitial decision statistic; 3) adjusting, in one or more of amplitudeand phase, the selected signal in step 2); and 4) forming the decisionstatistic based on the adjusted selected signal.
 9. The invention asrecited in claim 8, wherein: for step (b), each preamble sequence isselected from the OGC set formed from first and second sequence vectors,wherein the OGC set is generated from the first sequence vector and acyclic shift matrix of the second sequence vector; and wherein the fasttransform is a fast orthogonal Gold code transform (FOGT) comprising thesteps of 1) multiplying the preamble signal with the first sequencevector and a forward permutation vector to generate a permuted preamblesignal, the forward permutation vector mapping between i) the cyclicshift matrix of the second sequence vector and ii) a matrix ofWalsh-Hadamaard sequences; and 2) applying a fast Hadamaard transform tothe permuted preamble signal to generate the set of index values.
 10. Apreamble detector for detecting one of a set of preamble sequences in aspread signal, the preamble detector comprising: a first correlatorcorrelating a received spread signal with sequences of a firstorthogonal Gold code (OGC) set in accordance with a first fast transformto provide a preamble signal; a second correlator correlating thepreamble signal with the set of preamble sequences in accordance with asecond fast transform method to generate a set of index values, whereineach index value of the set of index values corresponds to a onepreamble sequence of the set of preamble sequences; a circuit forming adecision statistic based on the set of index values; and a selectorselecting, as the detected one of the set of preamble sequences, apreamble sequence corresponding to the decision statistic; wherein thecircuit forming the decision statistic comprises: a first magnitudedetector forming an initial decision statistic based on a relativemaximum index of the set of index values; a signal selector selecting asignal generated by the preamble signal combined with the preamblesequence corresponding to the relative maximum index of the initialdecision statistic; a coherent detector adjusting, in one or more ofamplitude and phase, the selected signal from the signal selector; and asecond magnitude detector forming the decision statistic based on theadjusted selected signal.
 11. The invention as recited in claim 10,wherein the first fast transform is a fast orthogonal Gold codetransform (FOGT), the first OGC set is generated from a first sequencevector and a cyclic shift matrix of a second sequence vector, and aforward permutation vector maps between i) the cyclic shift matrix ofthe second sequence vector and ii) a matrix of Walsh-Hadamaardsequences; and wherein: the first correlator comprises: a multipliermultiplying the received spread signal with the first sequence vectorand the forward permutation vector to generate a permuted sequencesignal; and a combiner applying a fast Hadamaard transform to thepermuted sequence signal to generate a set of correlated signals, thepreamble signal selected as one of the set of correlated signals basedon a predetermined decision criterion.
 12. The invention as recited inclaim 10, wherein: the set of preamble sequences is selected from asecond OGC set formed from first and second sequence vectors, the secondOGC set generated from the first sequence vector and a cyclic shiftmatrix of the second sequence vector; and the second fast transform is afast orthogonal Gold code transform (FOGT); and wherein: the secondcorrelator comprises: a multiplier multiplying the preamble signal withthe first sequence vector and a forward permutation vector to generate apermuted preamble signal, the forward permutation vector mapping betweeni) the cyclic shift matrix of the second sequence vector and ii) amatrix of Walsh-Hadamaard sequences, and a combiner applying a fastHadamaard transform to the permuted preamble signal to generate the setof index values.
 13. The invention as recited in claim 10, wherein theset of preamble sequences is selected from a set of Walsh-Hadamaardsequences, and the second fast transform is a fast Hadamaard transform.14. The invention as recited in claim 10, wherein the received spreadsignal is a burst of a random-access channel in a code-division,multiple-access communication system.
 15. The invention as recited inclaim 10, wherein the coherent detector includes a channel estimator fori) estimating a channel response from the preamble signal, and ii)forming a de-rotation signal from the preamble signal, and a combinerfor combining the de-rotation signal with the preamble signal forcoherent sequence detection.
 16. The invention as recited in claim 10,wherein the coherent detector includes a sequence generator, thesequence generator employing the initial decision statistic to locallygenerate a corresponding preamble sequence; and a combiner combining thelocally generated preamble sequence with the preamble signal forcoherent sequence detection.
 17. The invention as recited in claim 10,wherein the preamble detector is embodied in an integrated circuit. 18.A preamble detector for detecting one of a set of preamble sequences ina spread signal comprising the: a first correlator correlating areceived spread signal with a set of orthogonal sequences to provide apreamble signal; a second correlator correlating the preamble signalwith one or more preamble sequences of an orthogonal Gold code (OGC) setin accordance with a fast transform to generate a set of index values,wherein each index value of the set of index values corresponds to a onepreamble sequence of the set of preamble sequences; a circuit forming adecision statistic based on the set of index values; and a selectorselecting, as the detected one of the set of preamble sequences, apreamble sequence corresponding to the decision statistic; wherein thecircuit forming the decision statistic comprises: a first magnitudedetector forming an initial decision statistic based on a relativemaximum index of the set of index values; a signal selector selecting asignal generated by the preamble signal combined with the preamblesequence corresponding to the relative maximum index of the initialdecision statistic; a coherent detector adjusting, in one or more ofamplitude and phase, the selected signal from the signal selector; and asecond magnitude detector forming the decision statistic based on theadjusted selected signal.
 19. The invention as recited in claim 18,wherein: each preamble sequence is selected from the OGC set formed fromfirst and second sequence vectors, wherein the OGC set is generated fromthe first sequence vector and a cyclic shift matrix of the secondsequence vector and the fast transform is a fast orthogonal Gold codetransform (FOGT); and wherein the second correlator comprises: amultiplier multiplying the preamble signal with the first sequencevector and a forward permutation vector to generate a permuted preamblesignal, the forward permutation vector mapping between i) the cyclicshift matrix of the second sequence vector and ii) a matrix ofWalsh-Hadamaard sequences; and a combiner applying a fast Hadamaardtransform to the permuted preamble signal to generate the set of indexvalues.
 20. The invention as recite in claim 19, wherein the preambledetector is embodied in an integrated circuit.